Dimensional parameters are of major importance for single-mode optical fiber characterization. Dimensional parameters include both geometrical/structural features of the fiber (core and cladding radius, maximum numerical aperture, etc.), and properties depending on the propagation (mode-field dimensions and shape). The former properties are obviously of fundamental importance as to design of the fiber, while the latter properties (mode parameters) are of major importance in the characterization as to the intended use, since they allow joint losses, microbending losses and coupling efficiency to be determined, thus allowing a correct connection dimensioning.
Different definitions have been given up to now for such mode parameters and, in particular, for the mode field radius.
A definition frequently applied in the measurements gives mode field radius as the mean square width of the near-field and as the inverse of the far-field width.
The near-field value gives information on joint losses in the case of angular misalignment and on microbending losses, while the far-field value gives information on joint losses due to transverse offset and on guide dispersion.
The application of such definitions requires a double series of measurements, namely a near-field and a far-field measurement, for fiber characterization. For this reason, standardizing authorities are oriented towards different definitions of the mode-field radius: among these definitions, the width w.sub.b of the gaussian souce which affords maximum power coupling into the fiber is of particular interest, since from this definition values are obtained which are a good compromise between near-field and far-field values.
Denoting by .phi.(r) the field associated with the fiber (r=radial coordinate), and by G(r, w)=exp (-r.sup.2 /2w.sup.2) the source field (w=Gaussian width), the value w.sub.b is the value which maximizes the field superposition integral ##EQU1## and is implicitely defined by the relation ##EQU2##
A measurement method of value w.sub.b is described e.g. in the paper entitled "Spot-Size Measurements For Single Mode Fibers--A Comparison of Four Techniques" by W. T. Anderson and D. L. Philen, Journal of Lightwave Technology, Vol. LT-1, N. 1, March 1983, pp. 20 and ff. According to this paper, near-field intensity scanning of a light-beam at the output of the fibre under test is effected and function G(r; w) which better approximates the experimental data is obtained by a computing system.
This known method presents two disadvantages, namely:
(1) intensity scanning is a rather long operation;
(2) complex processings of the experimental data are necessary to obtain w.sub.b value, and hence rather sophisticated computing means are required.